Solvent Process

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The solvent process utilizes solvable salts or compounds that can be dissolved by acids as the raw materials; they are then mixed in water or other solvents to be uniform solutions. Then the solvents are evaporated by heating evaporation, spraying dryness, flaming dryness, or cooling dryness. Finally, the heat decomposition reactions result in the nanoparticles/nanocapsules or nanocomposites. Besides the solvent evaporation method, the emulsion techniques, such as partial-microemulsion, double-microemulsion, pressure homogenization-emulsification, or modified spontaneous emulsification solvent diffusion methods, can be categorized as solvent methods. The solvents used vary from water, to organic solvents, like acetone, ethanol, chloroform, benzene, 2-propanol, 2-methoxyethanol, 1,2-dimethoxyethane, N, N-diethylaniline, N, N'-dimethylformamide, trihexylamine, ammonium carbonate, ammonia solution, etc. The polymers, like poly(D,L-lactic acid), poly(N-vinyl-2-pyrrolidone), poly(lactide-co-glycolide), poly-2-caprolactone, poly(y-benzyl L-glutamate), poly(ethylene oxide), poly(ethylene glycol), etc., can be employed for this purpose, especially for coating the biomaterials. The solvent process has been used for encapsulation of not only metal nanoparticles, but also biomedical materials, such proteins, cyclosporin A, triptore-lin, Rolipram, insulin, vitamin E, and so on. The solvent process is a simple and cheap method which is suitable for industrial production of uniform nanoparticles/nanocapsules in large batches.

Entirely biodegradable poly(D,L-lactic acid) nanoparticles coated with albumin were prepared by the solvent evaporation technique [301]. Their degradative properties were investigated in simulated gastric and intestinal fluids.

A synthetic route was developed for the preparation of clay intercalated palladium catalysts [302]. A toluene-rich bulk liquid phase was equilibrated with a swollen, ethanol-rich interlamellar phase having a volume of about 0.9 cm3/g organoclay. Introduction of Pd2+ acetate into the organoclay suspension led to the generation and deposition of a metallic palladium dispersion in the interlamellar space. This system behaved as a versatile nanophase reactor, wherein the ethanol functioned as both solvent and reducing agent. The mean diameter of monodispersed Pd nanoparticles could be controlled from 17 to 30 A in a one-step reaction by changing the amount of protective polymer, poly(N-vinyl-2-pyrrolidone), and the kind and/or the concentration of alcohol in the solvent [303]. The Pd nanoparticles had fcc structures like that of bulk Pd, although the lattice constant increased with a decrease in the particle size.

The preparation and use of iron-based catalysts having performances very similar to those of NiMo/KB-based catalysts were described [304]. The iron-based catalysts were prepared by impregnating soluble iron salts onto KB.

The poly(lactide-co-glycolide) nanospheres were designed to deliver proteins for extended periods of time [305]. A water-in-oil-in-water (w/o/w) emulsion technique was modified. A study was performed to evaluate how the solvent elimination procedure, the copolymer type (different molecular weight and containing either free or esterified carboxyls), and the surfactant Poloxamer 188 affected the properties of the nanoparticles.

The long-term stability of cyclosporin A-loaded nano-particle suspensions, stored at 8 and 25 °C, was evaluated [306]. The stability of freeze-dried samples was investigated. Nanoparticles of poly-2-caprolactone, a biodegradable polymer, were obtained by a modified nanoprecipitation method. A central composite experimental design was used to investigate the simultaneous effect of technological factors (temperature of the aqueous phase and needle gauge) and formulation variables (volume of acetone and the amount of polymer and surfactant). The effect of these variables on the stability of the 100-220 nm particles was evaluated.

Silica-copper composite powders with high surface areas of about 200-400 m2/g were synthesized by the controlled hydrolysis/polymerization of sodium metasilicate (Na2SiO3) and copper nitrate [Cu(NO3)2- 3H2O] via partial-microemulsion and double-microemulsion processes at 28 °C [307]. Each microemulsion system consisted of sodium 1,4-bis(2-ethylhexyl) sulfosuccinate and sodium dodecyl sulfate, cyclohexane, and an aqueous solution of sodium metasilicate or copper nitrate. The partial-microemulsion method produced silica-copper oxide composites consisting of nanoparticles ranging from 20 to 50 nm with a uniform elemental distribution.

The preparation and the characterization of poly(lactic acid) nanoparticles containing protein C, a plasma inhibitor, were reported. nanoparticles were prepared by the doubleemulsion method (w/o/w), using methylene chloride as an organic solvent and polyvinyl alcohol or human serum albumin as a surfactant [308, 309]. The influence of experimental constraints such as sonication and organic solvent on protein C activity was evaluated.

Diblock copolymers composed of poly(y-benzyl L-gluta-mate) as the hydrophobic component and poly(ethylene oxide) as the hydrophilic component were obtained by the polymerization of y-benzyl L-glutamate N-carboxy-anhydride, initiated by the primary amino end group of the a-methoxy-omega-amino poly(ethylene oxide) [310, 311]. Nanoparticles were formed from an organic solution of the block copolymers by the diafiltration method.

An oligonucleotide drug was encapsulated within poly(lactide) microparticles with high encapsulation efficiencies at high theoretical drug loadings by the solvent evaporation method [312]. With the conventional w/o/w method, the encapsulation efficiency decreased with increasing internal water content, increasing stirring time prior to filtration of the microparticles, and increasing drug loading. The encapsulation was improved by replacing methylene chloride with ethyl acetate, by using micronized drug powder instead of an internal aqueous phase or by adding electrolytes or non-electrolytes to the external phase.

The imaging of ultrafine Au, Pd, CdS, and ZnS particles prepared in reverse micelles was studied by atomic force microscopy [313]. Mica substrates, derivatized with a mono-layer of amine or thiol-terminated silanes, were used to immobilize the particles. The incorporation of CdS nano-particles, as prepared in reverse micellar systems, into silica matrices, via a sol-gel process, was investigated [314]. The silica colloids containing CdS nanoparticles were prepared via the surface modification of CdS nanoparticles using 3-mercaptopropyltrimethoxysilane in reverse micelles. The recovery and dispersion of the nanoparticles was performed in an appropriate organic solvent, followed by a sol-gel process, using tetramethyl orthosilicate or tetraethyl orthosilicate in MeOH or EtOH solution. The size and morphology of the resulting silica colloids containing CdS nano-particles was controlled by changes in the silica source and the organic solvents.

An electrochemical procedure, based on the dissolution of a metallic anode in an aprotic solvent, was used to obtain silver nanoparticles ranging from 2 to 7 nm [315]. The influence of the different electrochemical parameters on the final size of silver particles was studied by using different kinds of counterelectrodes. The effect of oxygen presence in the reaction medium and the type of particle stabilizer employed were investigated.

The first step toward hydrophobic, polymer-based nanospheres for gene delivery is to encapsulate and release plasmid DNA. However, encapsulating large hydrophilic molecules in very small nanospheres is difficult. For example, maximizing protein and peptide as well as small molecule encapsulation requires adjustments in pH or addition of excipients to charge neutralize, and make less hydrophilic, the compound to be encapsulated. A cationic lipid was used to load and release plasmid DNA from nanospheres made by the phase inversion/solvent diffusion method [316].

The structural and magnetic properties of two Fe2O3-SiO2 nanocomposites, containing respectively 16.9 and 28.5 wt% Fe2O3, were investigated [317]. The nano-composites were synthesized by a sol-gel method, using ethylene glycol as a solvent, and heating the gels gradually to 900 °C. The procedure allows one to obtain y-Fe2O3 nano-particles homogeneously dispersed in the amorphous silica matrix.

Triptorelin is a decapeptide analog of luteinizing hormone releasing hormone, currently used for the treatment of sex-hormones-dependent diseases. The triptorelin-loaded nanospheres were prepared, which were useful for transdermal iontophoretic administration [318]. Nanospheres were prepared with the double-emulsion/solvent evaporation technique. The effect of three parameters on the encapsulation efficiency was determined: the role of the pH on the internal acid external aqueous phases.

The preparation of well-size-controlled colloidal gold nanoparticles in organic solvent was presented [319]. After the preparation of well-size-controlled aqueous colloidal gold particles, the solvent was changed to an organic one. This technique was required to enable a chemical reaction between gold particles and hydrophobic molecules, since a colloidal gold solution was typically prepared in water using a reduction process. The stability of the gold particle suspension was investigated, and it was found that the stability decreased in the sequence of water, ethanol, chloroform, and benzene solution.

Composite materials containing amorphous Cu nano-particles and nanocrystalline Cu2O embedded in polyaniline matrices were prepared by a sonochemical method [320]. These composite materials were obtained from the sonica-tion of Cu2+ acetate when aniline or 1% v/v aniline-water was used as solvent. Mechanisms for the formation of these products were proposed. The physical and thermal properties of the as-prepared composite materials were presented.

The preparation of nanoparticles as a potential drug carrier and targeting system for the treatment of inflammatory bowel disease was investigated [321]. Rolipram was chosen as the model drug to be incorporated within nanoparticles. Pressure homogenization-emulsification with a microflu-idizer or a modified spontaneous emulsification solvent diffusion method was used.

Di-n-hexadecyldithiophosphate (DDP)-coated ZnS nano-particles and noncoated ZnS nanoparticles were chemically synthesized [322]. The antiwear ability of the DDP-coated ZnS nanoparticles and noncoated ZnS nanoparticles as additives in liquid paraffin were evaluated on a four-ball machine; the morphologies of the worn surfaces were examined.

Nanoparticles of tin disulfide were prepared by the solvent-thermal synthesis method using different precursors

[323]. The products were of hexagonal SnS2 phase, which were laminar particles with the average size of about 150 nm.

Pd Nanoparticles were synthesized by reduction of palladium acetate by ethanol in systems containing tetrahydrofuran (THF) as dispersion medium and tetrado-decylammonium bromide (TDARr) surfactant as stabilizer

[324]. The polar phase (ethanol) acted at the same time as the reducing agent. THF/TDABr/H2O inverse microemul-sions containing micelles of various sizes were prepared, and the structure of complex liquids was studied by density measurements.

Nickel colloids were prepared by codeposition of the metal with several organic solvents: acetone, ethanol, 2-propanol, 2-methoxyethanol, and 1,2-dimethoxyethane at 77 K [325]. The stabilities of the colloids and fine powders were measured. The metallic films and active solids were obtained by evaporation under vacuum at room temperature.

A type of composite particle consisting of a zinc sulfide (ZnS) core and a silica (SiO2) shell or vice versa was reported [326]. Both kinds of morphologies were created using a seeded growth procedure using monodisperse seeds on which homogeneous layers with a well-defined thickness were grown.

Size-monodisperse metal nanoparticles were prepared via hydrogen-free spray pyrolysis [327]. Size-monodisperse pure copper particles are formed from metal salt precursors in a spray pyrolysis process using ethanol as cosolvent, thus avoiding the addition of hydrogen or other reducing gases.

Calix[4]resorcinarene-derived surfactants were highly effective at stabilizing metal nanoparticles of different sizes, creating opportunities to fabricate well-defined nanostruc-tures with size-tunable materials properties [328]. The resorcinarenes had a critical role in the dispersion of nanoparticles under various solvent conditions and in the robustness of the protective surfactant layer. Magnetic cobalt particles stabilized by resorcinarenes self-assemble into nanostructured "bracelets" in toluene. Resorcinarene surfactants can promote the self-organization of gold nano-particles as large as 170 nm into two-dimensional arrays.

The hybrid materials consisting of nonagglomerated iron oxide particles hosted in silica aerogels were investigated

[329]. The composite material can be produced as a monolith, in any shape, and with different dilutions of the iron oxide phase. Two sol-gel chemistry routes were followed: a solution of Fe(NO3)3-9H2O was added either to the silica gel or to the initial sol; the iron salt provided the water required for the gel polymerization. To obtain monolithic aerogels, the gels were dried by hypercritical solvent evacuation.

Silver and gold nanocrystals, sterically stabilized with dodecanethiol ligands, were dispersed in supercritical ethane

[330]. Nanocrystal dispersibility was measured as a function of solvent density—with pressures and temperatures ranging from 138 to 414 bar and 25 to 65 °C, respectively. Dis-persibility depended strongly on the nanocrystal size and solvent density.

Iron oxide aerogels were synthesized from tetram-ethoxysilicon(IV) or tetraethoxysilicon(IV) and iron nitrate using an acid-catalyzed solution sol-gel method combined with subsequent extraction of the alcoholic solvent with supercritical CO2 [331]. The main parameters varied in the sol-gel synthesis were the type of N-base used as the gelation agent (N, N-diethylaniline, trihexylamine, ammonium carbonate, ammonia), the concentration of the iron precursor, and the water content. The silicon precursor was prehy-drolyzed to improve its reactivity.

Insulin and insulin/poly(ethylene glycol)-loaded poly(L-lactide) nanoparticles were produced by gas antisolvent CO2 precipitation starting from homogeneous polymer/protein organic solvent solutions [332]. Nanospheres with smooth surface and compact internal structure were observed by scanning electron microscopy. The nanospheres presented a mean particle diameter in the range 400-600 nm and narrow distribution profiles.

Biodegradable polymer nanocapsules containing the lipophilic sunscreen, Parsol MCX, as the oil core were prepared by solvent displacement [333]. The influence of polysorbate 85 (P-85) and poloxamer 188 (P-188) as stabilizing agents, the Parsol MCX loading capacity, and the pho-toprotective potential of the formulations were investigated. The formation of nanocapsules was due to an interfacial instability arising from rapid diffusion of the solvent across the interface.

In2S3 nanoparticles, short nanowhiskers, nanorods, and finger-structure nanocrystals with stoichimetric composition and high quality were prepared by a solvent-reduction route [334]. The reagent concentrations, solvent, and reaction temperature played important roles in the shape control.

Polygonal (mainly triangular) silver nanoprisms were synthesized by boiling AgNO3 in N, N-dimethyl formamide, in the presence of poly(vinylpyrrolidone) [335]. Although during the synthesis a mixture of nanoprisms and nanospherolds was formed, the latter can be removed through careful cen-trifugation. The UV-vis spectra of the nanoprisms displayed an intense in-plane dipolar plasmon resonance band, as well as weak bands for in-plane and out-of-plane quadrupolar resonances. The nanoprisms were also stable in other solvents, such as ethanol and water, and solvent exchange led to strong shifts of the in-plane dipole plasmon band.

An understanding of the mixing and characterization of nanosized powders was gained [336]. Three different nano-sized material systems were selected based on their physical and chemical properties. Mixing experiments of the selected nanopowders were performed using a variety of environmentally friendly dry powder processing devices and the rapid expansion of supercritical CO2 suspensions, compared with solvent-based methods coupled with ultrasonic agitation.

The formation of silver nanoparticles in the presence of various macrocyclic thiol compounds in N, N'-dimethylformamide solution was investigated in-situ through visible spectrophotometric and photon correlation spec-troscopic measurements [337]. While temperature, solvent nature, and concentration of thiol compounds were important parameters in the process of metal cluster and particle generation, the chemical structure of the thiol capping molecules also played a crucial role in determining the average particle size. The existence of a macrocyclic effect was reported on silver particle formation by using two types of thiol macrocyclic compounds, thiolated S-cyclodextrins and thiolated cavitands [337]. Perthiolated S-cyclodextrins were found to be more efficient as a capping ligand than monothiolated S-cyclodextrins.

Core-shell type nanoparticles of poly(L-lactide)/poly (ethylene glycol) diblock copolymer were prepared by a dialysis technique [338]. Their size was confirmed as 40-70 nm using photon correlation spectroscopy. The H-1-nuclear magnetic resonance (NMR) analysis confirmed the formation of core-shell type nanoparticles and drug loading. The particle size, drug loading, and drug release rate of the poly(ethylene glycol) nanoparticles were slightly changed by the initial solvents that were used.

A chemical reduction route at room temperature was described to synthesize nanocrystalline CdS, ZnS, and CdxZn1-xS [339]. Anhydrous CdCl2, or ZnCl2, S, and KBH4 powders reacted at room temperature in various organic solvents, and the effect of the solvent on the quality of the nanoparticle product was investigated. Among the solvents used, tetrahydrofuran was shown to produce the highest quality single-phase nanoparticles.

The D-a-tocopheryl polyethylene glycol 1000 succinate (vitamin E TPGS) was applied as surfactant stabilizer to fabricate paclitaxel-loaded poly(lactide-co-glycolide) nanospheres in the solvent evaporation/extraction technique [340]. Encapsulation efficiency and in vitro release were measured by the high-performance liquid chromatography.

The preparation of capped metal oxide nanoparticles through the hydrolysis of metal salts was made arduous by the difficulty of dissolving long organic chain capping agents in water. By performing the reaction in propylene glycol under reflux, instead of water, one can hydrolyze FeCl3 in the presence of n-octylamine to obtain (repeatedly) soluble, monodisperse (around 5 nm) y-Fe2O3 particles displaying a tendency to aggregate into superlattices [341].

A liquid-crystal system was used for the fabrication of a highly ordered composite material from genetically engineered M13 bacteriophage and ZnS nanocrystals [342]. The bacteriophage, which formed the basis of the self-ordering system, was selected to have a specific recognition moiety for ZnS crystal surfaces.

Surface interaction of gold nanoparticles with solvents and functionalized organic molecules was probed using the changes in the surface plasmon absorption band [343]. A redshift in the surface plasmon band was observed with an increase in solvent dielectric constant for solvents that do not complex with the metal surface. A plot of the square of the observed position of the surface plasmon bands of Au nanoparticles in these solvents as a function of medium dielectric function showed a linear dependence.

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